1. Field of the Invention
The invention relates to a process of lowering the temperature of a gas stream to perform an intermediate step and then raising the temperature of the resultant gas stream. More specifically, this invention relates to recirculating the heat removed from the reactant gas stream to be used to increase the temperature of the product gas stream, thus minimizing the amount of heat rejected from the system and increasing the overall efficiency of the system.
2. Background of the Invention
Fuel cells generate power by facilitating the movement of charged species across electrical gradients. The source of these charge carriers can be hydrogen gas, or conventional hydrocarbon feedstocks reformed to hydrogen (H2) and carbon monoxide (CO) synthesis gas.
Fuel processing is an essential component of fuel cell systems that use conventional hydrocarbons. For solid oxide fuel cells (SOFCs), operating temperatures range between approximately 800° C. and 1000° C. Within this temperature range, heat from the fuel cell exhaust can be readily used to reform hydrocarbons to syngas (H2+CO).
Fuel cell systems utilizing fuel reformers require sulfur removal as part of their fuel processing, as none of the systems tolerate appreciable levels of sulfur in the fuel gas. There are two broad approaches to removing the sulfur from the hydrocarbon fuel supply: remove it before reforming the fuel, or remove it from the syngas produced by reforming. If utilized before conversion to syngas, the method must remove any sulfur form present in the feedstock. One technique uses hydrotreatment to convert the various sulfur compounds into hydrogen sulfide (H2S) for subsequent removal prior to reforming.
If the sulfur is removed after reforming, the sulfur will already be in the form of hydrogen sulfide.
Techniques to remove hydrogen sulfide are well established for temperatures between approximately 200° C. and 400° C. But these temperatures are much lower than the above stated 800-1000° C. temperatures used for fuel processing and fuel cell operation. This disparity presents a dilemma for thermal integration of fuel cell systems.
There are currently no known methods to remove H2S from syngas to desired levels of less than 1 ppmv (part per million by volume) at fuel cell operating temperatures without losing large quantities of heat from the system. Rejected heat penalizes the system efficiency; therefore, it must be minimized.
Heat-recirculating concepts have been applied to the opposite thermal problem of adding thermal energy to heat-recirculating combustion burners. In heat-recirculating burners, super-adiabatic temperatures are produced to allow the combustion of low-heating value fuels, enabling combustion to occur with a minimal amount of heat addition. A description of how heat recirculation, without mixing of products and reactants, could be used to burn mixtures of fuels having very low heating values was presented in: Lloyd, S. A., and Weinberg, F. J., “A Burner for Mixtures of Very Low Heat Content,” Nature, Vol. 251, 47-49 (1974). S. A. Lloyd, and F. J. Weinberg, “Limits to Energy Release and Utilisation from Chemical Fuels,” Nature, 257, 367-70 (1975).
Microscale burners, constructed in two-dimensional or three-dimensional “Swiss Roll” configurations, utilize heat recirculation to achieve sustainable combustion via super-adiabatic temperatures. This is shown in L. Sitzki, K. Borer, E. Schuster, and P. D. Ronney, “Combustion in Microscale Heat-Recirculating Burners,” The Third Asia-Pacific Conference on Combustion, Jun. 24-27, 2001.
A need exists in the art for a method to remove sulfur compounds from fuel cell syngas to the desired levels (<˜1 ppmv) without rejecting appreciable quantities of thermal energy. The method should lower the gas temperature in reformed gas to enable conventional sulfur removal in fuel cell operation while rejecting as little heat as possible. Also, the method should be simple to implement, requiring only rudimentary material and training.